Ubuntu just recently released the Developer Preview of Ubuntu Touch OS which would compete with Android, Open WebOS, as open source platform for smart phones and tablets. Ubuntu will however bring the much preferred Linux OS to the devices unlike Android which runs on top of Android. As the Ubuntu Touch is just in developer preview, a small range of decides are only supported at the moment. But they will likely grow with the continued development of the platform.
Galaxy Nexus(maguro), Nexus 4 (mako), Nexus 7 (grouper), and Nexus 10 (manta) are the devices that are currently working fine with Ubuntu Touch which can be downloaded and used for testing and other devices are a work in process. Developers are able to contribute by providing flash able images to the device they are working on. Ubuntu is currently building these previews based on Quantal, and will shift to Raring soon.
Check out the list of supported devices here:
If you are interested in the development of Ubuntu Touch or want Tyo be updated if your device will be updated in the list, I suggest you bookmark the link. Developers like pawitp are already working on older devices like Samsung Galaxy S so if your devices have better specs and developer interest then you can expect a Ubuntu Touch preview on your device as well.
Let us knotter what you think in the comments.